我怎么知道需要哪个隐式?
How do I know, which implicit that is needs?
我有一个工作代码:
final case class Services[F[_]](c: Client[F], fooUrl: String)
(implicit cf: ConcurrentEffect[F]) {
private val dsl = Http4sDsl[F]
import dsl._
def routes: HttpRoutes[F] = HttpRoutes.of[F] {
case GET -> Root / "call" =>
c.get(fooUrl) { res =>
Applicative[F].pure {
Response[F]().withEntity {
res
.body
.through(text.utf8Decode)
.map(msg => "Forwarded through Boo" |+| msg)
}
}
}
}
当我删除隐式导入 implicit cf: ConcurrentEffect[F]
时,编译器会报错:
[error] /home/developer/scala/boo-service/src/main/scala/com/sweetsoft/Services.scala:29:20: could not find implicit value for parameter instance: cats.Applicative[F]
[error] Applicative[F].pure {
[error] ^
[error] /home/developer/scala/boo-service/src/main/scala/com/sweetsoft/Services.scala:26:48: could not find implicit value for evidence parameter of type cats.Defer[F]
[error] def routes: HttpRoutes[F] = HttpRoutes.of[F] {
[error] ^
[error] two errors found
我怎么知道缺少 class implicit cf: ConcurrentEffect[F]
类型?
我认为如果您查看 cats-effect 库的来源,您会得到答案:
让我们转到 ConcurrentEffect 定义:
trait ConcurrentEffect[F[_]] extends Concurrent[F] with Effect[F] {...
嗯,现在让我们看看并发:
trait Concurrent[F[_]] extends Async[F] {...
异步:
trait Async[F[_]] extends Sync[F] with LiftIO[F] {...
下一个:
trait Sync[F[_]] extends Bracket[F, Throwable] with Defer[F] {
现在我们可以得出结论,ConcurrentEffect
扩展了 Defer
同样尝试对 Applicative
自己做 ;)
要快速处理此类错误 - 我认为您必须了解扩展层次结构 - 这样您才能快速找出您需要的类型
我有一个工作代码:
final case class Services[F[_]](c: Client[F], fooUrl: String)
(implicit cf: ConcurrentEffect[F]) {
private val dsl = Http4sDsl[F]
import dsl._
def routes: HttpRoutes[F] = HttpRoutes.of[F] {
case GET -> Root / "call" =>
c.get(fooUrl) { res =>
Applicative[F].pure {
Response[F]().withEntity {
res
.body
.through(text.utf8Decode)
.map(msg => "Forwarded through Boo" |+| msg)
}
}
}
}
当我删除隐式导入 implicit cf: ConcurrentEffect[F]
时,编译器会报错:
[error] /home/developer/scala/boo-service/src/main/scala/com/sweetsoft/Services.scala:29:20: could not find implicit value for parameter instance: cats.Applicative[F]
[error] Applicative[F].pure {
[error] ^
[error] /home/developer/scala/boo-service/src/main/scala/com/sweetsoft/Services.scala:26:48: could not find implicit value for evidence parameter of type cats.Defer[F]
[error] def routes: HttpRoutes[F] = HttpRoutes.of[F] {
[error] ^
[error] two errors found
我怎么知道缺少 class implicit cf: ConcurrentEffect[F]
类型?
我认为如果您查看 cats-effect 库的来源,您会得到答案:
让我们转到 ConcurrentEffect 定义:
trait ConcurrentEffect[F[_]] extends Concurrent[F] with Effect[F] {...
嗯,现在让我们看看并发:
trait Concurrent[F[_]] extends Async[F] {...
异步:
trait Async[F[_]] extends Sync[F] with LiftIO[F] {...
下一个:
trait Sync[F[_]] extends Bracket[F, Throwable] with Defer[F] {
现在我们可以得出结论,ConcurrentEffect
扩展了 Defer
同样尝试对 Applicative
自己做 ;)
要快速处理此类错误 - 我认为您必须了解扩展层次结构 - 这样您才能快速找出您需要的类型